Compost Ratio Calculator
Work out the carbon-to-nitrogen balance of your pile and what to add.
This tool was built by Serge, MSc., a plant biologist and environmental scientist with a BSc in Plant Biology and an MSc in Environmental Biology and Biogeochemistry. My research focused on how climate warming affects soil carbon cycling in boreal forest ecosystems. These tools are for educational purposes and give practical guidance, not professional advice.
Good compost depends on the ratio of carbon to nitrogen in the materials you add. Composting microbes work fastest near 30 parts carbon to 1 part nitrogen by weight. Add each material below with its weight, and this tool calculates the combined C:N of your mix and tells you whether to add more browns or greens.
Please add at least one material with a weight above zero.
How this works
Every material has its own C:N ratio. The tool multiplies each material's weight by its stored carbon and nitrogen fractions, adds the carbon across all materials, adds the nitrogen across all materials, then divides total carbon by total nitrogen. This gives the true weighted C:N of the whole mix, which is more accurate than counting buckets of browns and greens. The target band is 25:1 to 35:1, with 30:1 as the centre.
Understanding your result
A result near 30:1 sits in the range where a pile heats and breaks down quickly without smelling. A high number, say 50:1 or more, means too much carbon, so decomposition stalls and the pile stays cold. Add nitrogen-rich greens such as grass clippings, vegetable scraps or fresh manure. A low number, say under 20:1, means excess nitrogen, which is lost as ammonia and causes a sharp smell. Add carbon-rich browns such as dry leaves, straw, cardboard or sawdust.
Important note
The per-material C:N values are published averages. Real materials vary with age, moisture and source, so treat the result as a guide for adjusting your mix, not an exact laboratory figure. The C:N of a pile also falls as it matures, finishing near 10:1, so a fresh mix should start higher.
Scientific basis
- Cornell Waste Management Institute, Compost Chemistry, Cornell University: compost.css.cornell.edu/chemistry.html
- University of California Cooperative Extension, Composting 101 (PDF): ucanr.edu
- Richard, T. and Trautmann, N., C/N Ratio, Cornell Composting in Schools: css.cornell.edu
Disclaimer: This calculator is for general educational guidance only and does not replace professional or laboratory assessment.